Technical Field
The invention relates to the field of heat resistant separation fabrics. The invention describes knitted heat resistant separation fabrics that are produced via warp knitting and that provide improved properties. The warp knitted heat resistant separation fabrics of the invention can be used advantageously in the production of glass products, e.g. as mould covering in the bending of car glass. The invention further relates to methods to produce such warp knitted heat separation fabrics.
Background Art
US2002/0115369 discloses a stretch fabric that has an elongation of at least 20 percent in the warp direction. The stretch fabric is provided with both of the stretchability and the stiffness and is composed of a Raschel warp knit fabric formed of non-stretchable fiber yarns having no rubbery elasticity, wherein loops of tuck warp are engaged with loops of a ground fabric structure. The stretch knit fabric is suitably used as a medical material for covering or bandaging a human body having curved or irregular portions, as a reinforcement material for a moulded object having curved or irregular figure or portions, as a stretchable bandage, as a casting tape or as a splint for orthopedic surgery use.
The use of knitted fabrics, e.g. comprising stainless steel fibers, as heat resistant separation covering layer on tooling (moulds, rings, rollers) in car glass manufacturing is known e.g. from WO00/40792A1.
AU2003212039 discloses a covering material, resistant to high temperatures and intended to be inserted between tools and plates of hot glass during glass heating, bending or tempering processes. The covering material comprises a textile fabric comprising poly(p-phenylene-2,6-benzobisoxazole) fibers. The fabric can be a woven fabric, a braided fabric, a weft knitted fabric or a warp knitted fabric. The latter can be foreseen with polygonal openings, such warp knitted fabric can be used on cover rings, which support a glass plate during tempering operations.
EP0972751 describes a covering which is resistant to high temperature and intended to be inserted between tools and plates of hot glass. The covering includes a warp knitted fabric. The knitted fabric is provided with openings of polygonal shape, obtained by the mutual transfer of stitches from one wale to the other.
Heat resistant separation fabrics used for mould coverings in car glass manufacturing need to answer a number of requirements. They need to be extensible in order to be able to be mounted on the mould. State of the art knitted fabrics can be draped on the bending mould, but a risk exists that the fabric is not draped correctly, e.g. caused by overstretching in a particular direction of the fabric, resulting in quality problems when using the mould. The fabrics need to have sufficient air permeability as well and a high lifetime.
Most important however, is the quality of the car glass that is manufactured. Especially the optical quality of the car glass is essential. No defects are allowed in the car glass and the quality of the car glass has to be high over a long lifetime of the heat resistant separation fabric. Examples of defects that can occur are so-called orange peel and pitting. Besides defects in the car glass due to yarn defects or knitting defects in the fabric, defects in the glass can be related to the fabric construction or composition, to the mounting of the fabric on the mould, to—e.g. local—insufficient air permeability as well as due to ageing of the fabric. It is generally accepted that complex interactions play between the factors that can cause such defects in the car glass.